This invention relates in general to anti-lock brake systems and in particular an algorithm for scheduling extra reapply pulses in a rear wheel antilock brake system.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. One type of ABS controls only the vehicle rear wheel brakes. Such a system is referred to as a RWAL in the following.
A typical prior art RWAL is illustrated at 10 in FIG. 1. As shown in FIG. 1, the RWAL 10 is installed on a vehicle having a hydraulic braking system consisting of a brake pedal 12 coupled to operate a dual reservoir master cylinder 14. When the vehicle operator depresses the brake pedal 12, the master cylinder 14 supplies hydraulic fluid under pressure from a front reservoir 14a through a hydraulic line 16a and from a rear reservoir 14b through a hydraulic line 16b to a conventional combination or proportioning valve 18. The combination valve 18 includes a first output line 18a adapted to supply hydraulic fluid at a first predetermined pressure to actuate a pair of vehicle front wheel brakes 19a and 19b. The combination valve 18 also includes a second output line 18b which supplies hydraulic fluid at a second predetermined pressure to actuate a pair of vehicle rear wheel brakes 20a and 20b. 
The RWAL 10 shown in FIG. 1 utilizes a control valve 21 to selectively control the application of pressure to the rear wheel brakes 20a and 20b when the system is in an anti-lock braking mode. The control valve 21 includes a normally open solenoid valve 22 connected between the line 18b and a line 24 which supplies pressurized brake fluid to the controlled rear wheel brakes 20a and 20b. During an anti-lock braking cycle, the normally open valve 22 isolates the rear wheel brakes 20a and 20b from the master cylinder 14 and is commonly referred to as an isolation valve. The isolation valve 22 also can be selectively opened to increase the pressure at the rear wheel brakes 20a and 20b. 
The control valve 21 also includes a normally closed solenoid valve 26, which is connected between the line 24 and a fluid accumulator 28. The normally closed valve 26 is commonly referred to as a dump valve. The dump valve 26 is selectively opened to reduce the pressure at the rear wheel brakes 20a and 20b by bleeding brake fluid from the rear wheel brakes to the accumulator 28. In the RWAL 10, the master cylinder 14 provides a source of pressurized hydraulic brake fluid during an anti-lock braking cycle, thus eliminating the need for a separate source of pressurized hydraulic fluid, such as a motor driven pump, which is usually included in a four wheel ABS.
The RWAL 10 further includes a computer control module 30 which is electrically connected to a wheel speed sensor 40. The control module 30 can be mounted directly upon the control valve 21 or located remotely therefrom. The control module 30 includes a RWAL microprocessor (not shown) which is programmed to control the RWAL 10 in accordance with a RWAL control algorithm and parameters permanently stored in a Read Only Memory (ROM). The RWAL microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL illustrated in FIG. 1 is included in U.S. Pat. Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the RWAL control module 30 continuously receives speed signals from the wheel speed sensor 40. The RWAL microprocessor monitors the speed signals for potential rear wheel lock-up conditions. When the vehicle brakes are applied and the RWAL microprocessor senses a first rear wheel speed departure, which is indicative of an impending wheel lock-up condition, the RWAL microprocessor is responsive thereto to close the isolation valve 22 to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. The RWAL microprocessor then selectively opens the dump valve 26 to reduce the pressure applied to the rear wheel brakes 20a and 20b and thereby correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve 22 to initiate a second wheel speed departure.
The operation of the RWAL 10 is illustrated by the graphs shown in FIG. 2. The upper solid curve labeled 60 represents the velocity of the rear wheels while the dashed curve labeled 61 represents the vehicle velocity. The operation of the isolation valve 22 and the dump valve 26 is illustrated by the curves labeled 62 and 63, respectively. The lower curve, which is labeled 64, shows the pressure applied to the controlled rear wheel brakes.
During an anti-lock braking cycle, the first and second wheel speed departures are labeled 60a and 60b, respectively. Following correction of the second wheel speed departure, which occurs at time t7, the rear wheel brake pressure is maintained at a constant level Pe. Following the above described wheel departure and recovery cycles, it is known to open the isolation valve by supplying a number of very short reapply pulses, which are labeled 62b in FIG. 2, to the valve solenoid coil. The reapply pluses are intended to compensate for the progressive decrease in the brake lining friction coefficient due to heating and the increase in road friction that often occurs as the vehicle speed decreases. The reapply pulses also compensate for progressive thermal expansion of the rear brake drums which would cause the rear brake pressure to gradually decrease unless more brake fluid is added to the rear wheel brake system.
This invention relates to a rear wheel anti-lock brake system having an algorithm which schedules extra reapply pulses.
Referring again to the drawings, there is illustrated in FIG. 3, a graph of vehicle velocity vs. time which illustrates a known method for the time spacing of the reapply pulses. For simplicity, only the end portion of anti-lock brake cycle curve is shown in FIG. 3. It is desirable to have the reapply pulses spaced further apart in time on a low mu road surface than on a high mu road surface. Accordingly, the RWAL control algorithm usually includes a binary decision subroutine for determining whether the vehicle is on a low or a high mu road surface. The control algorithm then implements a difference in the time spacing of the reapply pulses that is in a ratio of five to three with the shorter time period being applied for a high mu road surface.
For the examples shown, the horizontal axis is divided into 100 millisecond increments. The upper vehicle velocity curve, which is labeled 70, is an example of the response of the control algorithm on a low mu road surface. For simplicity, a straight line is used for the upper vehicle velocity curve 70. A reapply pulse is generated every 500 milliseconds, as indicated by vertical marks on the upper curve 70.
The lower vehicle velocity curve in FIG. 3, which is labeled 75 is an example of the response of the algorithm on a high mu road surface. For simplicity, a straight line is used for the lower vehicle velocity curve 75. For illustrative purposes, it is assumed that the initial vehicle speed, Vi, is the same for both examples. A reapply pulse is generated every 300 milliseconds, as indicated by vertical marks on the lower curve 75.
For the examples shown in FIG. 3, the RWAL control algorithm results in seven reapply pulses being generated when the vehicle is on a low mu road surface while only four reapply pluses are generated on a high mu road surface. It would be desirable to increase the number of reapply pulses on the high mu road surface to further enhance the braking of the vehicle.
The present invention contemplates scheduling the reapply pulses at predetermined decrements in vehicle velocity during the portion of the anti-lock brake cycle that the rear wheel brake pressure is being held substantially constant. The predetermined decrements are trimable to allow adjustment of the braking cycle for the specific vehicle.
The present invention is contemplated for use with an anti-lock brake system for a vehicle having at least one wheel brake connected to a master cylinder. The anti-lock brake system includes an isolation valve connected between the master cylinder and the controlled wheel brake and a dump valve connected to the rear wheel brake. Additionally, the system has a speed sensor for monitoring the vehicle speed. The system further includes a controller electrically coupled to the isolation dump valves and the speed sensor. The controller being operative, upon detecting a potential lock-up condition of the vehicle wheel associated with the controlled wheel brake, to selectively operate the isolation and dump valves to correct the potential wheel lock-up condition. Following correction of said potential lock-up condition, the controller is further operative to selectively open the isolation valve upon the vehicle speed being decremented by a predetermined amount to generate a reapply pulse.
The invention further contemplates that the controller selectively opens the isolation valve generate the reapply pulse after a predetermined time period has elapsed following the correction of the potential wheel lock-up condition. In the preferred embodiment, the controlled wheel brake is a rear wheel brake.
It is further contemplated that the controller is operable to generate a plurality of reapply pluses, one of the reapply pulses being generated every time said vehicle speed is decremented by said predetermined amount.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.